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FOREST RESEARCH NOTES
U. S.DEPARTMENT OF AGRICULTURE
CALIFORNIA FOREST AND RANGE
EXPERIMENT STATION *
STEPHEN N. WYCKOFF, Director
June 27, 1950
WET WATER FOR FOREST FIRE SUPPRESSION^/
Wallace L. Fons , Mechanical Engineer
In recent years wetting agents have been recommended by manu-
facturers and others for increasing the efficiency of water for fire
suppression. Many local fire-protection agencies have made tests
comparing wetting agent solutions with plain water for flame suppres-
sion and mop-up under field conditions. In general, their results and
conclusions indicated the need for comprehensive tests to determine
the types of wetting agents best suited for suppression and the best
technique for application of chemically treated water, called wet water.
Such comprehensive research was begun in 1948 by the Engineering
Department at the University of California at Los Angeles. Technical
supervision of the research was administered by the Division of Forest
Fire Research, California Forest and Range Experiment Station. In 1949
the Station took over all phases of the investigative work.
At the outset it was realized that a great deal of fundamental
research would be necessary before specifications on all phases of
suppression could be developed. For instance, those physical properties
of water and chemicals accounting for the suppression action were not
known. The research program was therefore divided into three parts:
(1) Determining the physical properties of plain water and wet water;
(2) investigating the mechanisms of fire suppression; (3) determining
the suppression effectiveness of wet water over plain water.
1/ Presented at: Annual meeting, Southern California Association
of Foresters and Fire Wardens, San Bernardino, Calif., April 7, 1950, and
Annual meeting, California Rural Fire Association, Merced, Calif., April
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The physical properties thought to be important in suppression
are: surface tension, penetration, surface spreading, and foaming.
Surface tension is a measure of the force required to hold a liquid
drop in a spherical form. For instance, mercury has a high surface
tension value (513 dynes/cm); water, a moderate value (72.8 dynes/cm);
and alcohol and petroleum, low values (approximately 25 dynes/cm). Most
wetting agents in concentration of one percent will reduce surface tension
of water to 30 dynes/cm. The more active the wetting agent is, the more
rapidly the surface tension of water is decreased. The recommended con-
centration for fire fighting is usually such as to reduce the surface
tension of water to about 35 dynes/cm.
When the surface tension of water is reduced, spreading , penetrating,
and foaming are increased. Tests of these physical properties were made
on water ana 14 commercial wetting agents. The tests show that:
1. Surface spreading of wet water on wood is 2 to 8 times
greater than water, depending on species of wood.
2. Penetration into wood of wet water is about 8 times
greater than water.
3. Penetration of wet water into charcoal is about 2/3 as
much as it is for wood.
4. Penetration and surface spreading of water on charcoal
is practically nil.
5. All wet water readily forms foam.
Corrosion tests with iron and galvanized iron shewed that wet-
ting agent solutions in general, unless they contain an inhibitor .
have higher corrosion rates than distilled water. With few exceptions
the rate of corrosion was greater for the galvanized iron. The corro-
sion of metals by wetting agent solutions may be wholly corrected by
the addition of small amounts of chemicals, known as corrosion inhibitors,
such as potassium dichromate and sodium nitrite. Tests showed that as
little as 100 parts per million, or about 2 ounces per 100 gallons of
water, of commercial granular potassium dichromate was effective in
reducing the corrosion of iron and galvanized iron by wetting agent
Before tests with fires were begun, some important questions
arose concerning the suppression action of water First, what is the
behavior of a water droplet when it strikes a burning wood surface?
Calculations revealed that water, sprayed on burning wood with its
surface reduced to charcoal, will impinge on the charcoal surface
having a temperature as high .as 1900° F. Second, what is the
theoretical minimum volume of water necessary to cool a unit volume
of burning wood below the rekindling point? Again, calculations showed
that a unit volume of water suddenly applied over the entire surface
will cool 300 volumes of burning wood below the kindling point; but if
the water is applied only to the frontal area of the burning wood, the
amount of water required is doubled. Third, does penetration cooling
have an advantage over surface cooling and vice versa? Results of cal-
culations indicated that there was no significant difference between
penetration and surface cooling.
Water may be used to an advantage in forest fire suppression
for: (l) Flame suppression; that is, application of water on the flame
to retard the spread of a given section of fire; (2) mop-up; that is,
application of water on a burned area to extinguish small isolated
flames and to cool glowing and charred material to a point at which
rekindling is unlikely; (3) pretreatment ; that is, application of water
on unburned fuels in advance of an oncoming fire to retard its rate of
spread or in holding a line from which to backfire. For each of these
uses, fire tests were conducted to compare the suppression effectiveness
of wet water ana plain water.
For flame suppression, a total of 93 model fires were burned on
which the effectiveness of 14 commercial wetting agents were tested.
Besides these laboratory tests, 66 field-fire tests were made, in which
three commercial wetting agents were tested. Each fire was allowed to
burn up to its maximum flame intensity before suppression was started.
The intensity of each fire was measured with a radiometer. From the
radiometer record of each fire, the intensity during suppression was
The experimental work established the superiority of wet water
over plain water in reduction of fire intensity. The superiority is
at its maximum when the liquid is first sprayed on the fire. Results
indicate that this superiority at the very beginning of spray applica-
tion on a section of flaming front is from three to four times greater
than at the end of application, when only isolated flames remain. For
the period of flame suppression of these experimental fires, the mean
superiority values of wet water over plain water are 1.70 for field
fires and 2.10 for model fires. Ordinarily, superiority is expressed
as a ratio of quantity of plain water used to quanitty of chemical
solution used. The superiority of wet water based on quantities was
1.25 for field fires and 1.55 for model fires.
However, in recommending wet water for flame suppression emphasis
is given to the superiority based on reduction of flame intensity. The
practical importance of this superiority, especially at the beginning
of suppression, is that it shows the decided advantage a hose operator
has in advancing on a fire front with wet water. This advantage should
permit an operator to knock down the flame intensity more quickly to a
point where he can move in and hold the spread of a fire. It is believed,
therefore, that the use of wet water in flame suppression should result
in controlling a greater number of fires as well as in controlling the
fires at an earlier period, thus reducing the size of the mop-up job.
Even where an abundant supply of water is available, use of wet water
in some instances may make the difference between success and failure
in controlling a forest fire.
In these tests the ratios of volume of burning "wood to volume of
wet water used for flame suppression were: field fires, 56 to 1; model
fires, 117 to 1. For fires suppressed with water the ratios were 45 to
1 for field fires and 76 to 1 for model fires. These ratios, when com-
pared with 300 tc 1 — the maximum possible, show that there is room for
improvement in techniques of application.
Experimental work has not yet advanced sufficiently to specify
what pressures and discharge rates to use for most efficient flame
suppression. It has been noted, however, that on the experimental
fires sprays were superior to straight streams primarily because of
their greater coverage.
To test the effectiveness of wet water in mopping up fires in
deep pine litter, a total of 108 test fires were burned. The technique
used in mopping up these fires consisted of first spraying the burned
area as rapidly as possible, suppressing all glowing, smoking, and burn-
ing material; yet not wasting water or wet water by wetting the area
unnecessarily. In this first application care was taken to hold the
nozzle above the surface of the burning litter at a distance which gave
minimum air entrainment and effective spray distribution with maximum
coverage. The first application was followed by an intermittent appli-
cation on those spots where smoke appeared. A quick--opening-and-closing
shut-off nozzle permitted closing the nozzle quickly when moving from one
smoldering spot to another and opening it only long enough to extinguish
the immediate small hot area.
The results of 82 of these fires show that the superiority based
on quantity of plain water used compared to quantity of wet water used
is about 1.29. Thus, by the addition of wetting agents at appropriate
concentration, a saving of 23 percent in the quantity of water can be
realized on a given mop-up job. As intermittent application "was used part
of the time in mopping up these fires, the superiority of wetting agent
solutions based on time of mop-up is only 1.15. This means a saving of
13 percent in time in mopping up a given burn. In practice the saving of
23 percent in quantity of water and 13 percent in mop-up time may appear
as not being significant; however, on a national scale or even on one lar£
burn such a saving can amount to a considerable sum of money.
Another factor of practical importance is that wet water In mop-up
has a distinct advantage in reducing the rekindling from hot spots and
glowing embers. For the mop-up tests in deep pine litter the ratio of
rekindled spots on plots suppressed with water to the rekindled spots on
plots suppressed with wet water was 1.43 tc 1. For comparable fuel types
one can expect approximately 30 percent fewer rekindlings on an area mopped
up with wet water.
Pre treatment experiments were made by spraying wet water and
plain water on litter fuels to determine whether wet water would
increase the length of time that wetting is effective. The results
of these experiments indicate that the length of time for sprayed
unburned litter fuels to dry is 50 percent longer for wet water than
for plain water. This is attributed to the greater surface spreading
and penetration of wet water.
Significant differences were found in physical properties among
the wetting agent solutions tested. However , no special meaning can
be attached to these differences because these same agents were not
significantly different in their effectiveness in the fire suppression
tests. It appears that the surface spreading, penetrating, and foaming
properties of all the wetting agents tested, when the agent is used so
as to reduce the surface tension of plain water by one-half, are within
that range which does not alter the effectiveness of a wetting agent
solution for fire suppression. The results indicate that on the basis
of penetration and surface spreading, any one of the hundreds of wetting
agents now on the market, if used at the proper concentration, would
yield suppression-effectiveness results comparable to those obtained
for the wetting agents tested.
Specific conclusions drawn from work performed to date are:
(l) Savings up to 2 3 percent in the volume of water required and 13
percent in time of mop-up can be obtained with wet water if applied
with reasonable efficiency; (2) rekindling is reduced as much as 30
percent on fires mopped up with wet water as compared with plain water;
(3) foaming appears to be a desirable property of wet water in mop-up
because it prevents channeling of the water; (4) wet water is markedly
superior to plain water in its ability to knock down flames quickly,
thus permitting access to a fire edge not otherwise accessible; (5)
dead fuels along the burning edge of a fire or along a backfire line
remain wet up to 50 percent longer when sprayed with wet water than
when sprayed with plain water; (6) fuels once treated with wet water
and allowed to dry may be sprayed later with plain water with results
comparable to an original spraying with wet water; (7) most wetting
agents increase the corrosive action of water, but this can be wholly
corrected by the addition of chemicals known as corrosion inhibitors.
In general, the results to date have shown that wetting agents
have a definite place in forest fire suppression. Certain information
on foaming and dermatological effects, however, is still needed to
permit the writing of definite specifications for the most desirable
type of wetting agents. Some advancement has been made on technique
of application for maximum effectiveness; but this knowledge is not
yet complete enough to satisfy all field conditions. Experiments are
still needed to determine the effect of form and pattern of spray on
fire suppression efficiency, for plain water, wetting agent solutions,
and other chemicals.